This invention relates to measuring fluid parameters in pipes and more particularly to measuring fluid composition, volumetric flow, or other fluid parameters using a flow meter or meters assisted by a venturi.
In many industries it is desirable to measure various parameters of fluids or fluid mixtures in pipes, including the temperature, pressure, composition (i.e., phase fraction, e.g., 10% water, 90% oil), flow rate, and the speed of sound (SOS) in the fluid or mixture. (As used herein, xe2x80x9cfluidxe2x80x9d may refer to a liquid or gas, and a xe2x80x9cfluid mixturexe2x80x9d may be mixtures of liquids or gases). Different sensor arrangements, referred to generically as xe2x80x9cflow meters,xe2x80x9d can be used to measure these parameters, such as those that are disclosed in the following U.S. patent applications, which are incorporated herein by reference in their entireties, and which may have issued as U.S. patent Ser. No. 09/740,760, filed Nov. 29, 2000; Ser. No. 09/344,070, filed Jun. 25, 1999; Ser. No. 09/346,607, filed Jul. 2, 1999; Ser. No. 09/344,093, filed Jun. 25, 1999; Ser. No. 09/345,827, filed Jul. 2, 1999; Ser. No. 09/519,785, filed Mar. 7, 2000; Ser. No. 09/346,606, filed Jul. 2, 1999; Ser. No. 09/346,604, filed Jul. 2, 1999; Ser. No. 09/346,605, filed Jul. 2, 1999; Ser. No. 09/344,094, filed Jun. 25, 1999; Ser. No. 10/010,183, filed Nov. 7, 2001; and Ser. No. 09/344,069, filed Jun. 25, 1999.
A flow meter typically comprises a sensor, a sensor array, or multiple sensor arrays. In many of these flow meters, the sensors may comprise fiber optic sensors, possibly incorporating fiber Bragg gratings (FBGs), which can be mounted or coiled around the pipe containing the fluid to be measured. Other flow meters allow optical devices or other sensing devices to be ported or placed within the pipe to make the required measurements. When one uses a fiber optic based flow meter, the fluid or mixture parameters may be measured without the need to xe2x80x9ctap inxe2x80x9d to the pipe, as many of these parameters may be sensed externally to the pipe though the means disclosed in these incorporated references. Often, these externally mounted sensors are xe2x80x9cpassivexe2x80x9d sensors in the sense that they do not require stimulating the fluid or mixture of interest by external means, but instead make the required measurements simply by sensing various naturally occurring fluid effects.
In the oil and gas industry, or comparable industries, it is desirable to measure, in situ, the flow produced from an oil well. Typically the produced fluid may be comprised of three components or phases, such as oil, water, and gas, which may additionally contain other components, such as solids (e.g., rocks or sand) or other liquid phases. In a production environment, it is often useful to determine the phase fraction, or composition, of the fluid being measured, as well as the speed of the flowing mixture.
Techniques for measuring fluid or mixture flow rate exist in the prior art. For example, in patent application Ser. No. 09/346,607, entitled xe2x80x9cFlow Rate Measurement Using Unsteady Pressures,xe2x80x9d filed Jul. 2, 1999, incorporated herein by reference in its entirety, there is disclosed a flow rate meter which preferably uses fiber optic sensors. These fiber optic sensors are disposed at two different axial locations along the pipe containing the fluid to be measured. Naturally occurring pressure disturbances in the fluid perturb the first sensor through the wall of the pipe, creating a time-varying pressure signal. When the pressure disturbances, or pressure field, moves from the first sensor to the second sensor, a similar pressure signal is measured. The two signals from the pressure sensor can then be cross-correlated using well-known techniques to determine the time shift in the pressure signals. This time delay, when divided by the axial distance of the sensor, can be used to determine flow rate. Optionally, the sensors may comprise filters capable of filtering out certain undesirable wavelengths, which may constitute a single sensor or multiple sensors.
Other flow rate techniques using venturis are also known in the art. For example, U.S. Pat. No. 5,591,922, entitled xe2x80x9cMethod and Apparatus for Measuring Multiphase Flow,xe2x80x9d issued Jan. 7, 1997, and which is incorporated by reference herein in its entirety. In the ""922 patent, a pair of venturis within a pipe are spaced from one another at an axial distance. As is well known, the venturi causes a pressure difference (xcex94P) at each venturi, which are measured. These differential pressure signals are cross-correlated to determine a time delay, which can then be divided by the axial distance to determine a flow velocity.
Flow meters for determining phase fraction (xe2x80x9cphase fraction meterxe2x80x9d) in a fluid mixture are also known in the art. For example, U.S. Pat. No. 6,354,147, entitled xe2x80x9cFluid Parameter Measurement in Pipes Using Acoustic Pressures,xe2x80x9d issued Mar. 12, 2002, which is incorporated by reference herein in its entirety, a spatial array of pressure sensors, preferably fiber optic sensors, are coupled to the outside of the pipe. These sensor produce pressure signals, which are used to determine the speed of sound of the mixture. Because the speed of sound of a given mixture can be calculated from the speed of sound of the components in the mixture, the measured speed of sound can be used to determine the phase fraction of the mixture.
Often these various types of flow meters will be used in conjunction with each other to measure various fluid parameters of the device. For example, a flow rate meter may be used on one section of the pipe, followed downstream by a phase fraction meter, or vice versa. Or, these flow meters may be combined into an integrated flow meter apparatus.
The accuracy of these and other prior art flow meters are generally enhanced when the fluid mixture being measured is relatively well mixed or xe2x80x9chomogenous.xe2x80x9d Relatively inhomogeneous mixtures, having larger unmixed portions of the components that constitute the mixture, may not provide suitable pressure disturbances (i.e., acoustic differences) that can be easily resolved by the pressure sensors that typical constitute a traditional flow meters. Additionally, prior art flow meters may have difficulties in measuring the parameters of fluid mixtures having more than two phases. The art would therefore benefit from ways to improve the performance of these and other traditional flow meter techniques.
A venturi-assisted flow meter arrangement is disclosed. The venturi is positioned in the pipe or conduit containing the fluid mixture to be measured upstream of the flow meter. The flow meter is preferably a flow rate meter and/or a phase fraction meter. When the fluid mixture is passed through the venturi, it is homogenized or mixed, which can increase the accuracy of the measurements made by the downstream flow meter. Additionally, the venturi can be used to compute the flow momentum of the fluid mixture, which may be used to calibrate or double check the operation of the flow meter, or allow it to compute the phase fraction for a three phase mixture.